This invention relates to reducing the flow of broadcast datagrams over a communication link, and more particularly, to a method and apparatus for reducing such datagrams while maintaining reception of all broadcast datagrams by local area networks (LANs) coupled to either end of the communication link.
LANs provide a method for connecting computers or other devices together to exchange data or to harness groups of computers together to apply their combined power to a single problem. (The terms device and node as used herein refer generally to any of a variety of systems which can be connected to a LAN, including computers, file servers, routers, and bridges.) Generally speaking, a LAN includes: 1) a high speed transmission medium, typically metallic or fiber optic, for connecting each of the devices to the LAN; 2) the ability to transmit a message on the transmission medium directed to a single device (known as "directed datagram"); and 3) a means known as "broadcast" in which all devices connected to the LAN medium can receive a message transmitted on the medium. A standard for the implementation of LAN devices and systems has been established by the Institute of Electrical and Electronic Engineers as IEEE Standard 802.
The physical length of the transmission medium and the total number of devices which can be connected thereto are typically limited on a LAN due to the physics of high speed transmission systems. Bridges and routers are devices used to connect multiple LANs to provide communications between individual LANs and to construct large networks that transcend the technical size limits of a single individual LAN. When the individual LANs to be interconnected are at geographically remote locations, bridges and routers are used in pairs, one at each site, to provide a path for data to flow from one LAN to another, with a lower speed communication link (e.g., a private long distance telephone line) between the bridge or router pair. Typically the data rates of the long distance communication link between the bridge or router pair is a fraction of the data rate of the LAN medium. The use of bridges and routers has been limited, however, due to the cost of these devices and the costs of the long distance communication link.
An all-digital telephone network, known as the Integrated Services Digital Network ("ISDN"), has become a potential substitute for the private long distance lines currently used by bridges and routers. ISDN provides relatively high speed digital transmission service on an "as needed" basis, and is different from LAN transmission media in that it is a switched transmission media which provides a point-to-point transmission service on an intermittent basis.
Devices on LANs typically use three kinds of messages to transport information: directed datagrams, multicast datagrams, and broadcast datagrams. Some devices on a LAN require a constant exchange of broadcast datagrams to maintain proper operation. The traffic load of these broadcast datagrams on private lines or switched lines (e.g., circuit switched lines or packet switched lines) can be a significant percentage of the traffic capacity of the lines.
Some devices on a LAN such as routers and file servers rely on a continuous stream of broadcast datagrams to exchange topology and other information for proper operation. Interruption of this flow for a prolonged period (e.g., 60 seconds or more) will result in undesirable behavior of these devices.
It is desirable to reduce the traffic of broadcast datagrams over a communication link between LANs without interrupting the flow of such datagrams to devices on the LANs which must receive the broadcast datagrams in order to operate properly.